fix headers to utilize include guards, else all hell breaks lose easily

when someone tries to use them.

cherry picked because github merge is a fucking retard
(cherry picked from commit 76e0b93999b38de024631d5c60b2c1b65f33a81f)
Merge branch 'sreich-add-header-guards-v2' of git://github.com/sreich/Chipmunk-Physics into sreich-sreich-add-header-guards-v2
cpVect.c inline fix
Merge pull request #42 from vinay/master

Fixed compiler error in cpVect.c.
TODOs
Merge branch 'master' of github.com:slembcke/Chipmunk-Physics
older todo changes
More todo changes
Another todo addition
todo idea
cleanup and documentation
Updating version numbers and naming
Updating version info
Forgot to add the new doc examples
GitHub link
License date change
minor docs type
Merge branch 'master' of github.com:slembcke/Chipmunk-Physics
Merge branch 'master' of github.com:slembcke/Chipmunk-Physics
fix type qualifiers